Certifications don't hurt in job searches but they are usually not the number one thing employers want either in Unix/Linux. Also if an employer DOES require certification it typically IS vendor specific. This is somewhat different than the Windows world where certified admins are a dime a dozen so you stick out like a sore thumb if you don't have the Windows certification.
I've been a professional UNIX admin since 1991 and started dabbling in Linux around 1998. However its only been in the last 3-4 years that it seems to be making inroads in the server market (at the low end - notwithstanding exceptions like Google) in UNIX shops. In Windows shops people are starting to talk about desktop replacements but I haven't heard of anyone making a major commitment that way. Despite having no certification in any of the *NIXes I've worked on it seldom seems to have been the deciding factor in my job searches. Number one factor is experience (usually with the specific distro/variant the employer uses). In fact I got one job a few years ago because I'd worked on the Sun E10K. Despite being up against a Solaris admin of some 15 years experience I got the job ONLY because of the specific hardware experience with only a couple of years Solaris which to me seemed very short sighted on the employer's part. (Not that it kept me from taking the job
If you're really keen on it why not go to one of the job sites and do a search for Linux and certification in the job postings to see which specific certifications come up most often as requested.
P.S. Monster.com is the one where I've gotten most hits in my past 2 job searches and where my current employer found me even though I wasn't looking for a job at the time. ComputerJobs.com is the one I liked searching on but for some reason never seemed to get many calls out of it. Dice was supposed to be really good but I found it not to be so great in number of postings and NEVER got any calls from it. Local newspaper job boards I'd recommend just because they have the local classifieds but I truly never got anything out of them. Yahoo hotjobs seems to have very low employer interaction based on the number of hits it displays.